It is generally known that a fractal structure enhances the hydrophilicity of a hydrophilic solid surface to change the solid surface to superhydrophilic, and, on the other hand, renders a hydrophobic solid surface superhydrophobic by increasing water repellency.
Patent Literature 1 (JP-A-2005-343016) discloses a superhydrophobic coated article having a superhydrophobic coating on the surface of a substrate wherein the superhydrophobic coating includes projections formed of masses of fine particles and a water-repellent film and wherein the projections are present in dispersed regions of the coating and the surface of the regions of the coating in which the projections are present has irregularities formed by the projections.
Specifically, a three dimensionally bonded colloidal silica, an alkylalkoxysilane and a fluorine-containing alkylalkoxysilane are mixed together to form a cohydrolyzate polycondensate of the water-repellent materials and the silicon oxide fine particles, and this product is applied as a water-repellent dispersion to glass by a flow coating method and is dried naturally to produce a superhydrophobic plate.
Patent Literature 2 (WO 2003/039856) discloses a superhydrophobic substrate including a base, an undercoating film with minute irregularities disposed on the surface of the base, and a water-repellent coating disposed on the minute irregularities of the undercoating film, wherein the surface profile of the water-repellent coating is defined by particulate projections and columnar projections having a greater height than the particulate projections as measured from the surface of the base.
Specifically, the substrate is produced as follows. A decamethylcyclopentasiloxane solution of tetrachlorosilane is prepared which serves as a coating liquid for the formation of a silica-based irregular undercoating film. Separately, a decamethylcyclopentasiloxane solution of a fluorine-containing alkylalkoxysilane is prepared as a water-repellent agent. First, the coating liquid for the formation of an irregular undercoating film is applied to the surface of an automobile window shield glass and is allowed to stand, and subsequently the water-repellent agent is applied and is allowed to stand. Thereafter, the water-repellent agent on the surface is completely washed away with ethyl alcohol, and the surface is naturally dried (without firing), thereby producing a water-repellent window shield glass.
Patent Literature 3 (JP-A-2004-137137) discloses an article coated with a silicon oxide-based coating having minute irregularities on the surface wherein the minute irregularities are composed of minute projections and columnar projections. It is disclosed that the coating having such a structure may be formed by the application of a coating solution of a chlorosilyl group-containing compound in a solvent based on a silicon oil.
Patent Literature 4 (JP-A-H08-40748) discloses a water-repellent glass that includes a glass substrate and, on the surface of the substrate, an underlayer and a water-repellent layer disposed on the underlayer wherein the underlayer is an oxide thin film or a mixed oxide thin film which exhibits at least one or more surface configurations selected from a micropitted surface, an irregular surface and a convex or protruded surface as formed without any surface treatment, and wherein the water-repellent layer is a thin film formed by the application of a water-repellent and oil-repellent liquid obtained by adding an acid to a mixture solution including at least a fluoroalkylsilane, antimony oxide-doped tin oxide particles, a silicone compound, water and an organic solvent, the acid being added in an amount of 5×10−4 mol to 2×10−2 mol per 1 mol of the fluoroalkylsilane.
Specifically, water-repellent glass is produced by steps in which the underlayer is formed from an underlayer-forming coating solution that is an isopropyl alcohol dispersion of a mixture including a silica sol (molecular weight: about 3,000) and a silica sol (molecular weight: about 100,000) in a specific molar ratio; and separately a mixture as the water-repellent and oil-repellent liquid that includes an ethanol solution of a silica sol (molecular weight: about 3,000), antimony oxide-doped tin oxide particles (particle diameter: 20 nm), isopropyl alcohol, a heptadecatridecylfluoroalkylsilane and an aqueous nitric acid solution is applied and is dried at 250° C.